Computer systems typically use inexpensive and high density dynamic random access memory (DRAM) chips for main memory. Most DRAM chips sold today are compatible with various double data rate (DDR) DRAM standards promulgated by the Joint Electron Devices Engineering Council (JEDEC). DDR DRAMs offer both high performance and low power operation by providing various low power modes. One of these low power modes is known as precharge power down. During precharge power down, all banks are precharged, an on-chip delay locked loop (DLL) can be disabled, an input clock can be turned off, and output buffers can be disabled to significantly reduce power consumption. Precharge power down is relatively quick to exit, and exit time may be speeded up by keeping the DLL running. Another low power mode is active power down. Active power down differs from precharge power down in that at least one bank remains active, i.e. it has an open row, and thus it allows faster exit than precharge power down. Yet another low power mode is self refresh mode. In self refresh mode, all banks are idle and all external clocking is removed, but the DRAM maintains its contents by refreshing all rows periodically using a built-in timer.
Modern DDR memory controllers maintain queues to store pending memory access requests to allow them to pick the pending memory access requests out of order to increase efficiency. For example, the memory controllers can retrieve multiple memory access requests to the same row in a given rank of memory from the queue out of order and issue them consecutively to the memory system to avoid the overhead of precharging the current row and activating another row. In addition to improving efficiency the memory controllers place one or more ranks of DDR DRAMs in low power states in anticipation that the power savings will outweigh the extra time required to enter and exit this state. Known memory controllers do not place particular ranks of DDR DRAMs in a low power state unless there are no pending memory access requests to that rank in the queue.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.